Micro-electro mechanical systems (MEMS) are a relatively recent development. Such MEMS are being used as alternatives to conventional electro-mechanical devices as actuators, valves, and positioners. Micro-electromechanical devices are potentially low cost, due to use of microelectronic fabrication techniques. Novel applications are also being discovered due to the small size scale of MEMS devices.
Many potential applications of MEMS technology utilize thermal actuation to provide the motion needed in such devices. For example, many actuators, valves and positioners use thermal actuators for movement. In some applications the movement required is pulsed. For example, rapid displacement from a first position to a second, followed by restoration of the actuator to the first position, might be used to generate pressure pulses in a fluid or to advance a mechanism one unit of distance or rotation per actuation pulse. Drop-on-demand liquid drop emitters use discrete pressure pulses to eject discrete amounts of liquid from a nozzle.
Drop-on-demand (DOD) liquid emission devices have been known as ink printing devices in ink jet printing systems for many years. Early devices were based on piezoelectric actuators such as are disclosed by Kyser et al., in U.S. Pat. No. 3,946,398 and Stemme in U.S. Pat. No. 3,747,120. A currently popular form of ink jet printing, thermal ink jet (or “bubble jet”), uses electrically resistive heaters to generate vapor bubbles which cause drop emission, as is discussed by Hara et al., in U.S. Pat. No. 4,296,421.
Electrically resistive heater actuators have manufacturing cost advantages over piezoelectric actuators because they can be fabricated using well developed microelectronic processes. On the other hand, the thermal ink jet drop ejection mechanism requires the ink to have a vaporizable component, and locally raises ink temperatures well above the boiling point of this component. This temperature exposure places severe limits on the formulation of inks and other liquids that may be reliably emitted by thermal ink jet devices. Piezoelectrically actuated devices do not impose such severe limitations on the liquids that can be jetted because the liquid is mechanically pressurized.
The availability, cost, and technical performance improvements that have been realized by ink jet device suppliers have also engendered interest in the devices for other applications requiring micro-metering of liquids. These new applications include dispensing specialized chemicals for micro-analytic chemistry as disclosed by Pease et al., in U.S. Pat. No. 5,599,695; dispensing coating materials for electronic device manufacturing as disclosed by Naka et al., in U.S. Pat. No. 5,902,648; and for dispensing microdrops for medical inhalation therapy as disclosed by Psaros et al., in U.S. Pat. 5,771,882. Devices and methods capable of emitting, on demand, micron-sized drops of a broad range of liquids are needed for highest quality image printing, but also for emerging applications where liquid dispensing requires mono-dispersion of ultra small drops, accurate placement and timing, and minute increments.
A low cost approach to micro drop emission is needed which can be used with a broad range of liquid formulations. Apparatus and methods are needed which combine the advantages of microelectronic fabrication used for thermal ink jet with the liquid composition latitude available to piezo-electro-mechanical devices.
A DOD ink jet device which uses a thermo-mechanical actuator was disclosed by T. Kitahara in JP 2,030,543, filed Jul. 21, 1988. The actuator is configured as a bi-layer cantilever moveable within an ink jet chamber. The beam is heated by a resistor causing it to bend due to a mismatch in thermal expansion of the layers. The free end of the beam moves to pressurize the ink at the nozzle causing drop emission. Recently, disclosures of a similar thermo-mechanical DOD ink jet configuration have been made by K. Silverbrook in U.S. Pat. Nos. 6,067,797; 6,087,638; 6,209,989; 6,234,609; 6,239,821; and 6,247,791. Methods of manufacturing thermo-mechanical ink jet devices using microelectronic processes have been disclosed by K. Silverbrook in U.S. Pat. Nos. 6,180,427; 6,254,793; 6,258,284 and 6,274,056. The term “thermal actuator” and thermo-mechanical actuator will be used interchangeably herein.
Thermo-mechanically actuated drop emitters are promising as low cost devices which can be mass produced using microelectronic materials and equipment and which allow operation with liquids that would be unreliable in a thermal ink jet device. Thermal actuators and thermal actuator style liquid drop emitters are needed which allow the movement of the actuator to be controlled to produce a predetermined displacement as a function of time. Highest repetition rates of actuation, and drop emission consistency, may be realized if the thermal actuation can be electronically controlled in concert with stored mechanical energy effects. Further, designs which maximize actuator movement as a function of input electrical energy also contribute to increased actuation repetion rates.
For liquid drop emitters, the drop generation event relies on creating a pressure impulse in the liquid at the nozzle, but also on the state of the liquid meniscus at the time of the pressure impulse. The characteristics of drop generation, especially drop volume, velocity and satellite formation may be affected by the specific time variation of the displacement of the thermal actuator. Improved print quality may be achieved by varying the drop volume to produce varying print density levels, by more precisely controlling target drop volumes, and by suppressing satellite formation. Printing productivity may be increased by reducing the time required for the thermal actuator to return to a nominal starting displacement condition so that a next drop emission event may be initiated.
Apparatus and methods of operation for thermal actuators and DOD emitters are needed which minimize the energy utilized and which enable improved control of the time varying displacement of the thermal actuator so as to maximize the productivity of such devices and to create liquid pressure profiles for favorable liquid drop emission characteristics.
A useful design for thermo-mechanical actuators is a layered, or laminated, cantilevered beam anchored at one end to the device structure with a free end that deflects perpendicular to the beam. The deflection is caused by setting up thermal expansion gradients in the layered beam, perpendicular to the laminations. Such expansion gradients may be caused by temperature gradients among layers. It is advantageous for pulsed thermal actuators to be able to establish such temperature gradients quickly, and to dissipate them quickly as well, so that the actuator will rapidly restore to an initial position. An optimized cantilevered element may be constructed by using electroresistive materials which are partially patterned into heating resisters for some layers.
A dual actuation thermal actuator configured to generate opposing thermal expansion gradients, hence opposing beam deflections, is useful in a liquid drop emitter to generate pressure impulses at the nozzle which are both positive and negative. Control over the generation and timing of both positive and negative pressure impulses allows fluid and nozzle meniscus effects to be used to favorably alter drop emission characteristics.
Designs which produce a comparable amount of deflection and a deflection force while requiring less input energy than previous configurations are needed to enhance the commercial potential of various thermally actuated devices, especially ink jet printheads. The shape of the thermo-mechanical bender portion of the cantilevered element may be optimized to reduce the affect of loading or liquid backpressure, thereby reducing the needed input energy.
The spatial pattern of thermal heating may be altered to result in more deflection for less input of electrical energy. K. Silverbrook has disclosed thermal actuators which have spatially non-uniform thermal patterns in U.S. Pat. Nos. 6,243,113 and 6,364,453. However, the thermo-mechanical bending portions of the disclosed thermal actuators are not configured to be operated in contact with a liquid, rendering them unreliable for use in such devices as liquid drop emitters and microvalves. The disclosed designs are based on coupled arm structures which are inherently difficult to fabricate, may develop post-fabrication twisted shapes, and are subject to easy mechanical damage. The thermal actuator designs disclosed in Silverbrook '113 have structurally weak base ends which are subjected to peak temperatures, possibly causing early failure.
Further, the thermal actuator designs disclosed in Silverbrook '453 are directed at solving an anticipated problem of an excessive temperature increase in the center of the thermal actuator, and do not offer increased energy efficiency during actuation. The disclosed actuator designs have heat sink components which increase undesirable liquid backpressure effects when used immersed in a liquid, and, further, add isolated mass which may slow actuator cool down, limiting maximum reliable operating frequencies.
Cantilevered element thermal actuators, which can be operated with reduced energy and at acceptable peak temperatures, and which can be deflected in controlled displacement versus time profiles, are needed in order to build systems that can be fabricated using MEMS fabrication methods and also enable liquid drop emission at high repetition frequency with excellent drop formation characteristics.